In an evolution process of Long Term Evolution (LTE), LTE Release 13 (Rel-13) has started launching research projects in September, 2014. An important project in Rel-13 is use of unlicensed carriers in an LTE system. By such a technology, the LTE system can use currently existing unlicensed carriers, so that a potential spectrum resource of the LTE system is greatly improved, thereby achieving lower spectrum cost for the LTE system.
In addition, an unlicensed spectrum also has many advantages, for example:
1: no charge/low cost (it is unnecessary to purchase the unlicensed spectrum, and the spectrum resource is cost-free);
2: low access requirement and cost (individuals and enterprises may all participate in deployment of the unlicensed spectrum, and equipment of an equipment manufacturer may freely access the unlicensed spectrum);
3: shared resources (when multiple different systems operate in the unlicensed spectrum or different operating companies of the same system operate in the unlicensed spectrum, some resource sharing manners may be considered to improve spectral efficiency);
4: a variety of radio access technologies (which comply with different communication standards, are difficult to cooperate, and have diversified network topologies);
5: a large number of radio access data transmission sites (there are a large number of users, great difficulties in cooperation, and high overhead for centralized management); and
6: a large number of applications (data shows that multiple services may be operated in the unlicensed spectrum, for example, Machine to Machine (M2M) and Vehicle to Vehicle (V2V)).
Law control requirements of some regions or states may be raised for the utilization of unlicensed carriers. For example, in Europe, when a system operates in unlicensed carriers, a Listen Before Talk (LBT) mechanism may need to be supported, and two frame structure requirements under an LBT requirement are further specified in the control requirements to support data transmission, i.e. Frame-Based Equipment (FBE) and Load-Based Equipment (LBE) manners respectively. For more details please refer to Chapter 4.9 of ETSI EN 301 893 V1.7.1 (2012 June).
A system based on FBE frame structure has the following characteristics:
simplicity for implementation is achieved, and a fixed time length is occupied every time;
even though an occupying data transmission site sends no data during the frame, other data transmission sites are forbidden to use this frame;
a probability of occurrence of an idle sub-frame in a frame is high (because the time length is not determined according to a load), and the idle sub-frame is easy to be preempted by a different system;
there is only one opportunity for Clear Channel Assessment (CCA) in a frame, and a time length of the entire frame may be wasted if the opportunity is missed;
when the data transmission sites are asynchronous, the CCA preemption may be easily interfered, that is, a preemption success probability is low; and
when the data transmission sites are synchronous, multiple adjacent data transmission sites are likely to succeed in CCA preemption at the same time and use the resource at the same time, thereby resulting in high interference.
A system based on LBE frame structure has the following characteristics:
a length of a frame is variable, an occupied time length is between 1 ms and 10 ms (according to the latest control requirements), and each preemption is determined by a data transmission site;
CCA may be performed in each sub-frame to increase a preemption opportunity;
Extended Clear Channel Assessment (eCCA) is supported to be executed to solve the problem that multiple adjacent data transmission sites simultaneously preempt and simultaneously use a resource;
the occupied time length is determined according to a load, so that fewer resources are wasted;
a timing-related mechanism (for example, Hybrid Automatic Repeat reQuest (HARQ), uplink scheduling, Acknowledgement (ACK), Primary Synchronization Signal/Secondary Synchronization Signal (PSS/SSS) and Single Frequency Network (SFN)) of existing LTE has a high potential modification requirement; and
there are no obvious differences in case of synchrony or asynchrony of the adjacent data transmission sites.
For the characteristics of LBE and FBE, designing a frame structure suitable for data transmission of an LTE system based on either LBE or FBE has certain disadvantages, so that it may be suggested to analyze these disadvantages for providing a data transmission solution suitable for the LTE system and meeting regional control requirements.